Phenotypic changes in Arabidopsis caused by expression of a yeast vacuolar Ca2+/H+ antiporter.

In plants, cytosolic Ca2+ levels are tightly regulated, and changes in cytosolic Ca2+ have been implicated in converting numerous signals into adapted responses. Vacuolar ion transporters are thought to be key mediators of cytosolic Ca2+ concentrations. In an attempt to interpret the role of vacuolar Ca2+ transport in plant processes, we have expressed the yeast vacuolar Ca2+/H+ antiporter, VCX1, in Arabidopsis and tobacco. This transporter localizes to the plant vacuolar membrane. VCX1-expressing Arabidopsis plants displayed increased sensitivity to sodium and other ions. These ion sensitivities could be suppressed by addition of calcium to the media. VCX1-expressing plants demonstrated increased tonoplast-enriched Ca2+/H+ antiport activity as well as increased Ca2+ accumulation. These results suggest that VCX1 expression in Arabidopsis could be a valuable tool with which to experimentally dissect the role of Ca2+ transport around the plant vacuole.

Expression of arabidopsis CAX2 in tobacco. Altered metal accumulation and increased manganese tolerance.

Metal transport from the cytosol to the vacuole is thought to be an important component of ion tolerance and of a plant's potential for use in phytoremediation. The Arabidopsis antiporter CAX2 (calcium exchanger 2) may be a key mediator of this process. CAX2 expression in yeast suppressed both Ca(2+) and Mn(2+) growth defects. A peptide-specific antibody to the antiporter reacted with a 39-kD protein from plant vacuolar membranes. Tobacco (Nicotiana tabacum) plants expressing CAX2 accumulated more Ca(2+), Cd(2+), and Mn(2+) and were more tolerant to elevated Mn(2+) levels. Expression of CAX2 in tobacco increased Cd(2+) and Mn(2+) transport in isolated root tonoplast vesicles. These results suggest that CAX2 has a broad substrate range and modulation of this transporter may be an important component of future strategies to improve plant ion tolerance.